In recent years, with the prevalence of mobile electronic equipment such as a notebook personal computer, a smartphone, portable game equipment, and a PDA, in order to make these kinds of equipment lighter in weight and enable the equipment to be used for a long period of time, there has been a demand for attaining reduction in size of a secondary battery used as a power source and highly enhanced energy density thereof.
In particular, there has been an increase in the utilization of a secondary battery as a power source for vehicles such as an electrically-powered car, an electric motorcycle, and an electric vehicle. Such a secondary battery used also as a power source for vehicles is required to operate even over a wide temperature range as well as being required to have highly enhanced energy density.
As the secondary battery, a nickel-cadmium battery, a nickel-hydrogen battery, and the like have hitherto been the mainstream, but there is a tendency for a lithium ion secondary battery to be increasingly used.
Usually, a binder for electrodes (hereinafter, sometimes referred to simply as a binder) is dissolved/dispersed in a solvent/dispersion medium to prepare a binder solution/binder dispersion, the solution/dispersion is mixed with an active material and a conductive additive to prepare mixture slurry for electrodes (hereinafter, sometimes referred to simply as slurry), the slurry is applied onto a current collector, the solvent/dispersion medium is removed by a method such as drying, and the active material, the conductive additive, and the current collector are bound to one another to produce an electrode of the secondary battery.
Although a fluorine-based resin such as polyvinylidene difluoride (PVdF) has hitherto been used as the binder for electrodes, there is a problem that use of an organic solvent causes an increase in the environmental load because the fluorine-based resin is in a state of being dissolved in an organic solvent to be used in a solution state.
As such, there have been proposed binder compositions for lithium ion secondary batteries such as carboxymethyl cellulose (CMC) and a styrene-butadiene rubber latex (SBR) and a crosslinked compound of polyacrylic acid substituted with an alkali cation and polyvinyl alcohol, which can be dissolved or dispersed in water, are inexpensive, and give little environmental loads (Patent Document 1 and Patent Document 2).